ON THE ORIGIN OF THE VERTEBRATE TYPE. 601 



Of course, if the rudimentary vertebrate apparatus thus grew 

 into, and spread over, a raolluscoid visceral system, the formation 

 of the notochord under the action of alternating transverse strains, 

 did not take place as suggested in 255 ; but it does not therefore 

 follow that its ditferentiation from surrounding tissues was not 

 mechanically initiated in the way described. For what was said in 

 that section respecting the effects of lateral bendings of the body, 

 equally applies to lateral bendings of the tail ; and as fast as the 

 developing tail encroached on the body, the body would become 

 implicated in the transverse strains, and the differentiation would 

 advance forwards under the influences originally alleged. Obviously, 

 too, though the lateral muscular masses would in this case have a 

 different history ; yet the segmentation of them would be eventually 

 determined by the assigned causes. For as fast as the strata of 

 contractile fibres, developing somewhat in advance of the dorsal 

 axis, spread along the sides, they would come under the influence 

 of the alternate flexions ; and while, by survival of the fittest, their 

 parts became adjusted in direction, their segmentation would, as 

 before, accompany their increasing massiveness. The actions and 

 reactions due to lateral undulations would still, therefore, be the 

 causes of differentiation, with which natural selection would co- 

 operate. 



